Purinergic Signalling in Pancreatic Islet Endothelial Cells - Extracellular ATP and Adenosine as Regulators of Endothelial Cell Function - page 211

and presumably also the other endocrine cells have contact with at least one
capillary [7].

12.3.4 Islet Vascular Anatomy

There is a direct and separate arteriolar flow to the islets (Fig. 12.1), implying that
the islets can regulate their blood perfusion separately from that of the exocrine
parenchyma. Normally islet blood flow is 5-10 times higher than that in the exocrine
parts [9]. Besides mediating transport of nutrients and hormones, the islet endothe-
lium also affects a number of processes (Table 12.1) including the differentiation
of endocrine islet cells during development [50, 54], and in adults the regulation of
β -cell proliferation [48]. It is likely that the basement membrane of islet capillar-

ies has a special role in this context, due to differences in molecular composition
and integrin expression compared to exocrine capillaries [49, 65]. It was recently
shown that humans have a fused basement membrane between the islet endothelial
and endocrine cells, constituting a blood-islet barrier. [84]. The functional impor-
tance of this is unknown. We have preliminary results suggesting that rodent islet
endothelium, like bone marrow and liver sinusoidal endothelium, produces the pro-
tein stabilin-1, a marker for fagocytotic endothelia [31]. This suggests that there is a
need for protection of the islet endocrine cells against direct contact with the blood
stream, although the mechanisms may differ between species.
Islet endothelial cells are difficult to study in vitro, due to dedifferentiation upon
culture [59, 72, 73]. It is known from transplantation experiments, that freshly
isolated islets are revascularized by endogenous endothelial cells [71]. However,
cultured islets require host endothelial cells to form a new vascular system.

Fig. 12.1 Schematic drawing of the vasculature in the pancreas. Arterial blood flow is separate
for the endocrine and exocrine parenchyma, and 5-10% is diverted to the islets. Note that the islets
comprise only 1% of the pancreas, which means that their blood perfusion is 5-10 times higher
than that of the exocrine gland. An unknown quantity, mainly in larger islets, flows through an
insulo-acinar portal system, before being emptied into veins. The blood flow regulation occurs in
the pre-capillary circulation